It is known to use auxiliary floating converter cells to improve the harmonic performance of high power and/or medium voltage converter systems. The floating converter cells are included into the output of a main converter forming a floating cell stage for improving the harmonic content of the voltages generated by the main converter. The floating converter cells may only deliver reactive power and may be designed such that no auxiliary transformer windings are required to provide active power to the floating converter cells. In this case, cell capacitors of the floating converter cells may be balanced using energy taken from the main converter.
There are several approaches, how to control the floating converter cells. For example, a pulse-width modulated controller may be used to control the switching behavior of the floating converter cells. It also may be possible to determine the switching states of the whole converter system in one single controller. Furthermore, common-mode currents that are created by the switching of the main converter may be used to control the capacitor voltage of cell capacitors of the floating converter cells.
EP 0 884 831 A2 describes a control method for the converter topology described in DE 196 15 855 A1 based on pulse-width modulation. In EP 0 884 831 A2, the capacitor voltage of auxiliary converter cells is controlled through rectifier circuits.
In EP 1 253 706 B1, floating converter cells without rectifier circuits are described. A model predictive type controller is used to control both the main inverter and the floating converter cells in a single control stage. Furthermore, it is assumed that the capacitor voltages are controlled such that the complete converter system is regarded as a multilevel converter with equally spaced voltage steps. The capacitor voltage balancing is done by using the common-mode currents that are generated by the switching process and choosing redundant states of the converter system such that these currents are generated in proportion to the capacitor voltage error.
In U.S. Pat. No. 7,825,540 B2, a converter system with a main converter and with floating converter cells is shown, which are controlled by pulse-width modulation.